1. Field of the Invention
The present invention relates generally to an automatic focusing apparatus, and more particularly, to an improvement of an automatic focusing apparatus for automatically matching the focus relative to an object in response to a video signal obtained from an image sensor, in an image sensing apparatus such as a video camera having an automatic focusing mechanism.
2. Description of the Background Art
Conventionally, in an automatic focusing apparatus used in an image sensoring apparatus such as a video camera, an approach utilizing a video signal itself obtained from an image sensor for evaluating the state of focus control has been developed. According to such an approach, a lot of good characteristics can be obtained. For example, there exists substantially no parallax. In addition, even if the depth of field is small and an object is located in the distance, the focus can be exactly matched. Furthermore, according to this approach, a specific sensor for automatic focusing need not be separately provided, so that the apparatus has a very simple mechanism.
A control method known as a hill-climbing servo system is one example of such a conventional focus control method utilizing a video signal, conventional has been conventionally known. The hill climbing servo system is described, for example, in U.S. Pat. Nos. 4,922,346 and 5,003,339. Briefly stated, a high frequency component of a video signal is detected every one field as a focus evaluating value, the focus evaluating value is always compared with a focus evaluating value detected one field before, and the position of a focusing lens is continuously slightly vibrated such that the focus evaluating value always takes the maximum value.
In the above described hill-climbing servo system, if only the slope of a focus evaluating value is detected, the focusing lens is not stopped in a defocused position by always driving the focussing lens in the direction of increasing the focus evaluating value, even if the object is changed, so that very good follow-up characteristics with respect to the object can be achieved.
FIG. 6 is a block diagram showing the whole structure of the hill-climbing servo system automatic focusing apparatus and FIG. 7 is a block diagram showing details of the focus evaluating value generating circuit of FIG. 6.
Referring to FIG. 6, the video camera includes a focusing ring 2 for moving forward and backward the focusing lens 1, a focusing motor 3 for driving focus ring 2, an iris mechanism 21 for adjusting exposure, and an image sensor circuit 4 including an image sensor (not shown) such as a CCD.
An image formed on a surface of the image sensor by focusing lens 1 is converted into a video signal by image sensing circuit 4, and applied to a focus evaluating value generating circuit 5. Referring to FIG. 7 which shows details of focus evaluating value generating circuit 5, a luminance signal component in the video signal applied from image sensing circuit 4 is applied to a synchronous separator 5a and to a highpass filter (HPF) 5c. When the luminance signal passes through highpass filter 5c, only the high frequency component is separated from the luminance signal, and is applied to a detector 5d. The high frequency component of the luminance signal is amplitude-detected by detector 5d, and the detected output is sampled and successively converted to a digital value by an A/D converter 5e.
Meanwhile, synchronous separator 5a separates a vertical synchronizing signal and a horizontal synchronizing signal from the applied luminance signal, and applies these to a gate control circuit 5b. Gate control circuit 5b sets a rectangular focusing area in a central portion of a picture in response to the applied vertical and horizontal synchronizing signals and to a fixed output of an oscillator (not shown). Gate control circuit 5b applies a signal for opening or closing a gate every field to gate circuit 5f such that passage of the high frequency component of the luminance signal which has been digitized is permitted only in the range of the focusing area. Gate circuit 5f may be provided anywhere before an integration circuit 5g, which will be described later.
Only the detected output corresponding to the focusing area is applied every field to integration circuit 5g by gate circuit 5f. Integration circuit 5g integrates the applied detected output field by field, and provides the result as a focus evaluating value (FEV) of the current field. In other words, in integration circuit 5g, digital integration, in which A/D converted outputs of the focusing area obtained in 1 field period are all added, is performed.
Focus evaluating value generating circuit 5 structured as described above constantly outputs the focus evaluating value of 1 field, and circuits in the succeeding stage start focusing operation by utilizing the focus evaluating value.
FIG. 8 is a graph showing the relation between the focus evaluating value and the lens position (distance between the lens and the object, that is, object distance) in the automatic focusing apparatus shown in FIG. 6. In FIG. 8, the abscissa represents the position of the focusing lens, and the ordinate represents the focus evaluating value.
Referring to FIGS. 6 and 8, immediately after an automatic focusing operation is started, the focus evaluating value corresponding to the first one field supplied from the focus evaluating value generating circuit 5 is applied to a maximum value memory 6 and an initial value memory 7 and held therein. Thereafter, a focusing motor control circuit 10 controls a focusing motor driving circuit 31 such that focusing motor 3 rotates in a prescribed direction. Then, a comparator 9 compares the initial focus evaluating value held in the initial value memory 7 and the current focus evaluating value provided from focus evaluating value generating circuit 5, and provides a comparison signal. Focusing motor control circuit 10 initializes direction of rotation of focusing motor 3 in response to the comparison signal.
More specifically, focusing motor control circuit 10 rotates focusing motor 3 in the above described predetermined direction until comparator 9 generates a comparison output of "large" or "small". If a comparison output indicating that the current focusing evaluating value is larger than the initial focus evaluating value held in initial value memory 7 is output from comparator 9, focusing motor control circuit 10 maintains the above mentioned prescribed direction of rotation. If a comparison output indicating that the current focus evaluating value is smaller than the initial focus evaluating value is obtained, focusing motor control circuit 10 reverses the direction of rotation of focusing motor 3.
Thus, initialization of the direction of rotation of focus motor 3 is completed, and from this time on, focusing motor control circuit 10 monitors the output from comparator 8. In order to prevent a malfunction due to noise of the focus evaluating value, comparator 9 is adapted not to generate the comparison output indicating "large" or "small" as long as the difference between the initial focus evaluating value and the current focus evaluating value does not exceed a predetermined threshold value.
Meanwhile, comparator 8 compares the maximum focus evaluating value so far held in maximum value memory 6 and the current focus evaluating value output from focus evaluating value generating circuit 5, and outputs two different comparison signals S1 and S2 indicating a mode (first mode) in which the current focus evaluating value is larger than the focus evaluating value held in maximum value memory 6 and a mode (second mode) in which the current focus evaluating value is decreased by more than a predetermined threshold value M. Here, when the current focus evaluating value is larger than the content of maximum value memory 6, the content of the maximum value memory is updated in response to the output S1 of comparator 8, so that the maximum value of the focus evaluating value so far is always held in the maximum value memory 6.
A motor position detector 30 detects amount of rotation of focusing motor 3, and generates a corresponding lens position signal. The lens position signal indicates the position (lens position) of lens 1 in the direction of the optical axis. The lens position signal is applied to lens position memory 13. Similar to maximum value memory 6, lens position memory 13 is updated such that the lens position signal at which the focus evaluating value attains the maximum is always held, in response to the output S1 from comparator 8.
As described above, focusing motor control circuit 10 monitors the output from comparator 8 while rotating the focusing motor 3 in the initially set direction, in response to the output from comparator 9. When comparison output S2 of the second mode indicating that the current focusing evaluating value decreases from the maximum focus evaluating value by more than the threshold value M is obtained from comparator 8, as shown in FIG. 8, focusing motor control circuit 10 reverses the direction of rotation of focusing motor 3. By this reverse rotation of focusing motor 3, the direction of movement of the lens switches from the direction toward the image sensor to the direction away from the image sensor, or vice versa.
After focusing motor 3 is reversed, a comparator 14 compares the content in lens position memory 13 corresponding to the maximum value of the focusing evaluating value with the current lens position signal supplied from motor position detector 30. When these two match, that is, when lens 1 returns to a position at which focus evaluating value attains the maximum, focusing motor control circuit 10 stops rotation of focusing motor 3. At the same time, focusing motor control circuit 10 provides a lens stop signal LS. In the above described manner, a series of automatic focusing operations are completed.
A memory 11 and a comparator 12 are provided for resuming an automatic focusing operation by focusing motor control circuit 10 if the focus evaluating value changes by more than a predetermined threshold value while the focusing lens is stopped. More specifically, a focus evaluating value at the time point when the automatic focusing operation by focusing motor control circuit 10 is terminated and lens stop signal LS is generated is held in memory 11. Comparator 12 compares the content of memory 11 with the current focus evaluating value supplied from focus evaluating value generating circuit 5, and if the difference therebetween exceeds the predetermined threshold value, it is considered that there has been a change in the object, and a signal indicating the change of object is applied to focusing motor control circuit 10. As a result, automatic focusing operation by focusing motor control circuit 10 is resumed, so that an automatic focusing operation following the change of the object can be achieved.
However, the above described automatic focusing apparatus suffers from the following disadvantageous.
First, since the rotational speed of the focusing motor 3 cannot be increased, it is difficult to achieve a high-speed automatic focusing operation.
Referring to FIG. 8, it is assumed that the automatic focusing operation is started in the state in which the lens is in a position A considerably spaced apart from an in-focus position P on the side of the object. In this case, focusing motor 3 rotates in the direction of increasing the distance between the lens and the object from the position A where the focus evaluating value is small and the object is significantly defocused, so that the focus evaluating value rises rapidly. As the lens reaches the vicinity of point P1, the rise of the focus evaluating value becomes moderate. Then, the lens passes through the in-focus position p and reaches a position P' of the lens where the focus evaluating value drops by more than the aforementioned threshold value M. Thereafter, the lens returns from the lens position P' to the in-focus position P and stops there.
It is required that such a series of automatic focusing operations are performed at high speed. However, when focusing lens 1 is moved at high speed by rotating focusing motor 3 at high speed, the following problem arises.
First, it is inevitable that overrun occurs due to the inertia of the motor itself when the motor is reversed when at focus lens the position P' or the motor is stopped at focus lens position P. The higher the speed of rotation of focusing motor 3 is, the larger the overrun becomes. As a result, the time is made rather longer until the lens reaches the in-focus position.
The conventional automatic focusing system is superior in its focusing precision and response to a wide variety of objects. However, whether the in-focus position P is reached or not is detected by the decrease of focus evaluating value. Unfortunately, since there is inevitably a delay in the focus evaluating value by the period from the time point when the light enters the image sensor until the end of integration in the focus evaluating value generating circuit 5, the focusing lens 1 will pass over the in-focus position principally. Accordingly, if the speed of moving the lens is increased, the amount of overrun of the focusing lens 1 would be significantly large. Therefore, blurring of the picture caused by the overrun is inevitable, particularly when the lens position is actuated at high speeds.
Thus, in the conventional automatic focusing apparatus, the speed of rotation of focusing motor 3 cannot be set very high. Consequently, so that a high-speed automatic focusing operation cannot be performed.
Furthermore, the conventional automatic focusing apparatus suffers from a disadvantage that the focus evaluating value changes because of interlaced scanning. More specifically, in the above described conventional automatic focusing apparatus, the position of the lens is controlled such that a focus evaluating value obtained from a level of high frequency component of the video signal is always the maximum. However, since the video signal is generally subjected to interlaced scanning, the positions of even and odd fields constituting one picture are shifted in the picture by one scanning line. Consequently, even if the same object is continuously picked up, the focus evaluating value fluctuates with every field, resulting in the disadvantage that the lens position at which the focus evaluating value attains the maximum cannot be definitely determined.
Further, in the above described automatic focusing apparatus, when the object does not change at all, in-focus state can be achieved by the above described focusing operation. However, in actual recording, the object may move forward, backward, left or right and the shape of the object itself may change, so that the focus evaluating value may change because of such movement, not because the movement of focusing lens 1.
If the object changes considerably, the direction of in-focus position P may be erroneously recognized at the initial stage, or comparator 8 may erroneously determine that the focus evaluating value is decreased by more than the predetermined threshold value M before the in-focus position P is reached, resulting in malfunction such as erroneous switching of the mode to the second mode and reverse rotation of focusing motor 3.
Meanwhile, when an iris of iris mechanism 21 adjusting the amount of incident light to the image sensor is opened/closed, the relation between the focus evaluating value and the lens position changes as shown in FIG. 8. In FIG. 8, when the iris of iris mechanism 21 is opened, that is, the F value is made smaller and the amount of incident light is not match intercepted, the focus evaluating value changes as shown by the solid curve of FIG. 8. When the same object is being picked up under the same condition with the iris closed, that is, with the F value increased, the focus evaluating value changes as represented by the dotted curve of FIG. 8. Therefore, the range of the lens position in which the focus evaluating value of a prescribed level can be obtained becomes wider if the iris is closed, and as a result, the depth of field becomes deeper.
If the iris is closed and the depth of field is deep, the curve of the focus evaluating value becomes moderate as can be seen from FIG. 8, and the rate of change of the focus evaluating value with respect to the amount of change of the lens position becomes smaller, resulting in higher possibility of the aforementioned malfunction.
If focus motor 3 is rotated at a high speed so as to make the amount of change of the focus evaluating value per 1 field sufficiently large, such a malfunction can be prevented. However, as described above, the higher the speed of rotation of focusing motor 3, the more overrun.